What is he best method to do a screen overlay with unreal engine 4

I would like to do a screen overlay (similar to fb messenger's chat heads) using Unreal Engine 4. Is there a way to do this? I don't mind it requiring c++ but would rather start with blueprints if possible and I would like to od so on android and iOS however android would be a great start. :) I don't require programing to be done for me but a few hints and a point in the right direction for further researh would be fantastic. :)